1. Field of the Invention
The present invention relates generally to a fuel supply apparatus, and in particular, to a fuel supply apparatus for an outboard motor.
2. Description of the Related Art
Generally, in an outboard motor, the interior of fuel piping that extends from the inside of an inboard fuel tank to the outboard motor is not allowed to be pressurized. Thus, fuel is first drawn up to the outboard motor by a low pressure fuel pump installed in the outboard motor, and is then pressurized by a high pressure fuel pump arranged in the outboard motor, so that the pressurized or high pressure fuel is supplied to a fuel injector unit. However, most of the inner space defined in the outboard motor is occupied by the engine itself. So in cases where the engine is stopped after warm-up operation, vapor which is bubbles of the evaporated fuel can be generated in the fuel in the fuel passages extending in the outboard motor up to the high pressure fuel pump due to the heat of the engine. If the high pressure fuel pump suctions in a large amount of vapor thus generated, vapor lock will occur. Vapor lock is a phenomenon in which vapor enters the fuel pump where it cannot be pressurized and the fuel cannot be discharged.
As a countermeasure against vapor lock, there is one described in a first patent document (Japanese patent application laid-open No. H8-312485). FIG. 7 is a piping diagram that shows a known fuel supply apparatus in an outboard motor. In this figure, a fuel tank 1 disposed on the bottom of a boat is connected to a low pressure fuel pump 3 disposed in a casing 2 of an outboard motor through fuel piping 4. The low pressure fuel pump 3 is connected to a volumetric chamber 6 through an inflow passage 5. A needle valve 8 cooperating with a float 7, a filter 9 and a Westco type high pressure fuel pump 10 are arranged in the volumetric chamber 6. The needle valve 8 is adapted to be opened and closed in accordance with the amount of fuel in the volumetric chamber 6 so as to adjust the fuel therein to a predetermined amount. The high pressure fuel pump 10 has a fuel pressure holding valve 10a attached thereto, which is adapted to be opened when the fuel discharge pressure becomes higher than the pressure in a high pressure passage 11 by a predetermined value or more. The high pressure fuel pump 10 is connected to a fuel injector unit 12 through the high pressure passage 11 and serves to pressurize the fuel in the volumetric chamber 6 thereby to supply it to the fuel injector unit 12. The fuel injector unit 12 injects the high pressure fuel thus supplied into an intake pipe 13 of the unillustrated engine in the casing 2. The operations of the high pressure fuel pump 10 and the fuel injector unit 12 are controlled by a drive unit 14. A pressure regulator 16 is connected to the high pressure passage 11 through a drain passage 15. The pressure regulator 16 is connected to the volumetric chamber 6 through a return passage 17, so that when the pressure in the high pressure passage 11 becomes equal to or higher than a predetermined value, the pressure regulator 16 is opened to permit the high pressure fuel in the high pressure passage 11 to return to the volumetric chamber 6, thereby adjusting the pressure in the high pressure passage 11 in an appropriate manner. At an upper location of the volumetric chamber 6, there is arranged a vapor discharge passage 19 that is in communication with the intake pipe 13 through a canister 18. The vapor accumulating in the upper location of the volumetric chamber 6 is collected in the canister 18 and is discharged to the intake pipe 13 upon starting of the unillustrated engine. That is, in this example, the volumetric chamber 6, the float 7, the needle valve 8, the canister 18, and the vapor discharge passage 19 together constitute a vapor liquid separation mechanism 20 that adjusts the vapor in the volumetric chamber 6 to a predetermined amount.
Next, FIG. 8 is a piping diagram that shows another known fuel supply apparatus for an outboard motor. As shown in this figure, a high pressure fuel pump 10 may be arranged outside of a volumetric chamber 6. In this case, the high pressure fuel pump 10 is connected to the volumetric chamber 6 through an outflow passage 21.
In the known fuel supply apparatuses of the outboard motors as described above, the amount of vapor generated changes depending on the amount of heat generated by the engine and the layout of the vapor liquid separation mechanism 20. Thus, variation is caused in the Air/Fuel mixture at the time of engine starting due to a change in the amount of vapor discharged into the intake pipe 13. In other words, in the known apparatuses, vapor is discharged into the intake pipe 13, so it is necessary to change the layout, etc., of the apparatus according to the specification of the engine so as to keep variations in the mixture to a small level, thus resulting in high costs due to an increase in the layout variation etc.